1. Field of the Invention
The present invention relates to a method and apparatus for acquiring a reference grating of a three-dimensional measurement system using moiré and, more particularly, to a method and apparatus for acquiring a correct reference grating of a three-dimensional measurement system using moiré to accurately measure the shape of a target object.
2. Background of the Related Art
In general, there are two techniques of measuring the three-dimensional shape of a free-form surface. A technique has been widely used which measures points on a curved surface one by one in a contact manner using a three-dimensional measuring device to measure the shape of the overall surface. However, disadvantageously, this method requires an excessively long measurement time. Accordingly, a non-contact type optical method referred to as a moiré method has been frequently used in recent days. This method has an advantage in that a measurement time is remarkably reduced as compared to the contact method using a three-dimensional measuring device.
Various studies have been carried out on moiré effect by many people since the moiré effect was initially proposed as a scientific tool by Lord Rayleigh in 1874. In particular, moiré became a useful tool for analyzing the strain of an object in mechanical engineering fields since it was used to measure the in-plane of an object.
Since Meadows, Takasaki et al. proposed that the moiré effect was used to measure the out-of-plane shape of an object in 1970, a three-dimensional shape measurement method using the moiré effect came into the spotlight. Moiré is divided into shadow moiré and projection moiré according to the method of forming a moiré pattern.
The moiré is required to form straight stripes on a target object at a predetermined interval and accurately move the straight stripes in order to acquire a moiré pattern having three-dimensional shape information. To achieve this, a conventional method projects a straight-line glass grating, which is obtained by engraving straight stripes at a predetermined interval on one side of glass with chrome, to a target object using a projection optical system.
Furthermore, straight stripes formed on the target object are moved at a predetermined interval using a straight-line glass grating actuator. When the straight-line glass grating is projected to the target object, stripes are formed on the target object. These stripes are curved according to the height of the target object.
When the target object on which the stripes are formed is overlapped with the straight-line glass grating, a moiré pattern can be obtained. This moiré pattern is a contour formed according to the height of the target object, and thus the moiré pattern is analyzed to measure the shape of the target object.
A phase shifting moiré measuring device which is a currently frequently used device using moiré includes a white light source, a condenser, a projection grating and a projection lens and projects the projection grating to an object to form deformed stripes. A grating actuator moves the projection grating by 3 to 5 steps at an equal interval. A CCD camera acquires a moiré pattern according to a reference grating identical to the projection grating, a relay lens and an imaging lens.
However, an image obtained by the aforementioned conventional moiré measuring device simultaneously includes a moiré pattern representing height information of a target object and the pattern of the reference grating placed in front of the CCD camera. Accordingly, an additional device for removing the image of the reference grating is required, and thus the structure of the measuring device becomes complicate.
To address and solve this problem, light is irradiated to a plane on which an object is not placed through a projection grating without using a reference grating, as illustrated in FIG. 1, to acquire a grating pattern (reference grating) as illustrated in FIG. 2. Then, the object is located and a moiré pattern of the object is obtained to measure the shape of the object. However, in the process of obtaining the reference grating, an invisible reference grating is acquired because of grating bending, noise caused by particles, and ununiform reference grating pitch due to unevenness of the plane to which light is irradiated, as illustrated in FIG. 2. When this reference grating is applied to measure the shape of the object, a measurement error is generated.